The present invention concerns a system for controlling a foundation brake of a vehicle. The present invention also concerns a method for protecting a foundation brake. The invention is intended for use particularly, but not exclusively, in trucks, buses and heavy good vehicles but is applicable to any other vehicle such as a passenger car.
The brake system of a vehicle usually comprises a foundation brake. The foundation brake is the basic drum or disc brake assembly fitted to each axle or wheel which produces the braking force necessary to bring the vehicle to a stop. A vehicle may further comprise an adaptive cruise control (ACC) device that automatically adjusts the vehicle's speed to maintain a safe following distance. An ACC device usually utilizes a radar, installed behind the grill of the vehicle, to detect the speed and distance of any vehicle ahead of it. If the lead vehicle slows down the ACC device sends a signal to the engine or braking system of the vehicle to decelerate. Then, when the road is clear, the ACC device re-accelerates the vehicle back to a pre-set speed.
If a foundation brake is used excessively, such as if it is applied several times over a prolonged period of time, fatigue or fading, i.e. gradual or sudden loss of braking power, may occur. If brake pads are used excessively their optimal working temperature is exceeded and their coefficient of friction decreases since, at a certain temperature, certain elements of a brake pad can start to melt and vapourise. The vapour becomes trapped between the brake pad and the rotor surface causing the brake pad to hydroplane on a boundary layer of vaporized friction material which acts like a lubricant. Additional brake pedal pressure is then required to slow down the vehicle, which increases braking temperatures and further destroys the brake pad, resulting in further losses of braking power. At temperatures of approximately 600-800° C. the friction coefficient of the friction material can be reduced by approximately 60%. At temperatures of approximately 900° C. brake pads can start to burn. Heat generated in the brake pads may be dissipated using a cooling system; however the amount of heat that may be effectively dissipated by the cooling system is limited.
In order to protect a foundation brake many vehicles are equipped with a supplementary auxiliary brake. US patent application no. 2004/0119333 discloses a system for controlling the brakes of a commercial vehicle. The system includes an ACC device that modulates an urgency signal based upon a hazard variable such as the relative speed of, and/or distance to a vehicle traveling in front of said commercial vehicle. The commercial vehicle comprises an electronically controlled brake system, which is designed to distribute a desired amount of braking force to a friction brake system, i.e. a foundation brake, and an auxiliary brake, namely a retarder. A retarder is a device that is permanently fitted to the vehicle's engine or transmission to augment the vehicle's braking capability during prolonged brake application. The electronically controlled brake system is designed in such a way that the distribution of the desired braking force to the friction brake system and the retarder is also based upon the urgency signal.
Such a supplementary auxiliary brake may be used to protect a foundation brake and give the driver greater control and improved brake performance. However, an auxiliary brake such as a retarder, increases the cost, weight and space requirements of a vehicle's brake system and decreases the fuel efficiency of the vehicle.
It is desirable to control the foundation brake of a vehicle so as to protect the foundation brake in a simple and cost-effective way.
According to an aspect of the present invention, a system comprises an ACC device and means to detect or predict excessive use of the foundation brake. The system also comprises means to disengage, i.e. de-activate, disconnect or switch off the ACC device on detection or prediction of excessive use of the foundation brake. By disengaging the ACC device and thus transferring control of the foundation brake solely to the driver of the vehicle in situations where an ACC device may cause a foundation brake to be used excessively, fading and fatigue of the foundation brake is reduced or avoided. An ACC device may therefore be used together with a foundation brake without damaging the foundation brake of a vehicle. Furthermore, a supplementary auxiliary brake or retarder can be used together with a foundation brake in an ACC device without damaging the foundation brake of a vehicle.
According to an embodiment of the invention the system comprises means to determine the temperature of the foundation brake i.e. it comprises means to measure or calculate the temperature of at least one part of the foundation brake or the temperature of a component in the vicinity of the foundation brake. The system comprises means to disengage the ACC device if/when the temperature of the foundation brake reaches or exceeds a predetermined temperature, such as a temperature in the range 300-600° C. Said predetermined temperature will vary depending on the type of brake and/or brake pads used.
According to another embodiment of the invention the system comprises means to determine a brake duration factor (BD) of the foundation brake during a predetermined time interval and means to disengage the ACC device if/when the brake duration factor of the foundation brake reaches or exceeds a predetermined brake duration factor maximum limit. Said brake duration factor (BD) is dependent on the use of the foundation brake, i.e. the brake duration, during a predetermined time interval.
According to a further embodiment of the invention the system comprises means to determine whether the vehicle is at a safe distance behind any object in front of said vehicle, such as another vehicle, and means to disengage the ACC device only if/when the distance between the vehicle and any object in front of said vehicle corresponds to, or exceeds a predetermined safe distance. Not only is the foundation brake thereby protected, but the safety of the driver of the vehicle and other road users is ensured. The magnitude of the safe distance depends on the speed of the vehicle and may be calculated by the inventive system or obtained from pre-computed values that are stored in the inventive system. According to an embodiment of the invention the system comprises means to decrease the speed of the vehicle until said predetermined safe distance has been achieved.
According to another embodiment of the invention the system comprises indication means to inform the driver that the ACC device is being or has been disengaged. Said indication means provides an optic and/or audio and/or tactile signal for example.
According to a further embodiment of the invention the system comprises means, such as a road surface inclination sensor or pre-programmed route profile information or a global positioning system (GPS), to determine whether the vehicle is approaching a situation in which the use of the ACC device may result in excessive use of the foundation brake; for example means to determine whether the vehicle is about to follow a lead vehicle down a long descent.
The present invention also concerns a method for protecting a foundation brake of a vehicle comprising an ACC device. The method comprises the steps of determining whether the foundation brake is being used excessively and disengaging the ACC device if this is the case.
According to an embodiment of the invention the method comprises the step of determining the temperature of the foundation brake, directly or indirectly, and disengaging the ACC device if said temperature reaches or exceeds a predetermined temperature.
According to another embodiment of the invention the method comprises the step of determining a brake duration factor (BD) of the foundation brake and disengaging the ACC device if said brake duration factor reaches or exceeds a predetermined brake duration factor limit. Said brake duration factor limit depends on the use of the brakes during a predetermined time interval. According to an embodiment of the invention, the brake duration factor BD when the vehicle is braking, i.e. the foundation brakes are used to brake the vehicle, is determined using the expression:
                              B          ⁢                                          ⁢                      D            ⁡                          (              t              )                                      =                                            ∫                              t                ⁢                                                                  ⁢                1                            t                        ⁢                                                  ⁢                          ⅆ              t                                +                      B            ⁢                                                  ⁢                          D              ⁡                              (                                  t                  1                                )                                                                        (        1        )            
The brake duration factor BD when the vehicle is not braking, i.e. the foundation brakes are not used to brake the vehicle, is determined using the expression:
                              B          ⁢                                          ⁢                      D            ⁡                          (              t              )                                      =                              a            ⁢                                          ∫                                  t                  ⁢                                                                          ⁢                  1                                t                            ⁢                                                          ⁢                              ⅆ                t                                              +                      B            ⁢                                                  ⁢                          D              ⁡                              (                                  t                  1                                )                                                                        (        2        )            
The constant a is less than or equal to 1. In this embodiment, BD is thus a variable that increases with slope 1 when the foundation brakes are used to brake the vehicle and decreases with slope a when the foundation brakes are not used.
According to another embodiment of the invention the brake duration factor (BD) is defined by the expression:
                              B          ⁢                                          ⁢                      D            ⁡                          (              t              )                                      =                              c            ⁢                                          ∫                                  t                  ⁢                                                                          ⁢                  1                                t                            ⁢                                                          ⁢                              ⅆ                t                                              +                      B            ⁢                                                  ⁢                          D              ⁡                              (                                  t                  1                                )                                                                        (        3        )            where c is a function dependent on one or more of the following: vehicle speed, brake pressure, brake disc temperature, ambient temperature, type of brake disc, type of brake pads.
According to a further embodiment of the invention the method comprises the step of determining whether the vehicle is at a safe distance behind any object in front of said vehicle and disengaging the ACC device if/when the distance between said vehicle and said object corresponds to, or exceeds a predetermined safe distance. The ACC device is therefore not disengaged automatically as soon as excessive use of the foundation brake is detected or predicted but at a time thereafter when it is safe to do so. According to a yet further embodiment of the invention the method comprises the step of decreasing the speed of the vehicle until a predetermined safe distance has been achieved and then disengaging the ACC device.
According to an embodiment of the invention the method comprises the step of indicating to the driver of the vehicle that the ACC device is being or has been disengaged by means of an optic and/or audio and/or tactile signal for example.
According to another embodiment of the invention the method comprises the step of reengaging the ACC-device manually or automatically once the temperature or the brake duration factor of the foundation brake has dropped to a threshold value for example.
The present inventions also concerns a computer program product that comprises a computer program containing computer program code means arranged to cause a computer or a processor to execute at least one of the steps of a method according to any of the embodiments of the invention, stored on a computer-readable medium or a carrier wave.
The present invention further concerns an electronic control unit (ECU) that comprises a computer program product according to the present invention.
The present invention also concerns a vehicle that comprises a system or an ECU according to any of the embodiments of the invention.
It should be noted that the drawings have not been drawn to scale and that the dimensions of certain features have been exaggerated for the sake of clarity.